1. Field of the Invention
The present invention relates to an information processing apparatus utilizing a physical phenomenon (a tunneling current, evanescent light, etc.) occurring when a probe is brought into proximity to a sample, and more particularly to a probe position controlling mechanism for controlling a distance between the probe and the sample.
2. Related Background Art
Recently, applications of memory materials are central to electronics industries including computers and associated equipment, video disks, digital audio disks, etc., and material development is in progress very actively.
Although performance required from the memory materials differs depending upon applications, a quick response speed of recording or reproduction is necessary and indispensable.
The majority of conventional memories were semiconductor memories and magnetic memories made of semiconductor materials or magnetic materials, but, with recent progress in the laser technology, cheap and high-density recording media are coming along as optical memories using an organic thin film of an organic dye, a photopolymer, or the like.
Developed these years on the other hand is a scanning tunneling microscope (hereinafter referred to as STM), which can directly observe electronic structures of surface atoms in a conductor [G. Binnig et al., Phys. Rev. Lett, 49, 57 (1982)]. The STM permits high-resolution measurement of real aerial image, irrespective of whether an object is of a single crystal or amorphous. In addition, the STM has an advantage that it can measure a sample at low power without damaging the sample by an electric current. Further, it can operate in the air, and can be used for a variety of materials. Therefore, a wide range of applications are expected for the STM.
The STM utilizes the phenomenon that a tunneling current flows when a metal probe (probe electrode) is brought to the distance of about 1 nm to an electrically conductive substance as applying a voltage between the probe and the conductive substance.
This current is very sensitive because it exponentially responds to a change in the distance between the probe and the substance.
When the substance is scanned with the probe while keeping the tunneling current constant, various information even concerning a total electron cloud in the real space can be read.
The resolution in in-plane directions by this is about 0.1 nm, and, therefore, applying the principles of STM makes high-density recording and/or reproduction possible substantially in the atomic order (the sub-nanometer order).
For example, the information processing apparatus as disclosed in the gazette of Japanese Laid-open Patent Application No. 61-80536 is arranged to remove atomic particles adsorbed on the surface of a medium with an electron beam or the like so as to write information and to reproduce the data with the STM.
Another method is also proposed in such an arrangement that a recording layer is made of a material having the memory effect for switching characteristics of voltage or current, for example a thin-film layer of one out of organic compounds having the conjugated .pi. electron system or chalcogenides and the STM is used to perform recording and/or reproduction [Japanese Laid-open Patent Applications No. 63-161552 and No. 63-161553].
Supposing the bit size of recording is 10 nm, this method enables large-capacity recording and/or reproduction of 1 Tbits/cm.
Further, a scanning mechanism of the probe electrode is one of a cantilever type (Japanese Laid-open Patent Application No. 62-281138). A plurality of cantilever mechanisms can be made of SiO.sub.2 in the size approximately of 100 .mu.m in length, 10 to 20 .mu.m in width, and 0.5 .mu.m in thickness on a silicon substrate, and writing and reading circuits are also integrated on the same substrate.
As apparent from such principles of STM, angstrom-level control is necessary for position control of probe in order to measure a surface structure of substance by the STM.
For that purpose, there have been developed actuators for control, such as those utilizing a piezoelectric device or those utilizing an electrostatic force. A cylinder type device or layer-built type device may be used as a device that utilizes a piezoelectric device. Further, there may be also used a lever type device such as a bimorph cantilever which includes a cantilever in which two layers of piezoelectric members and an electrode are alternately laminated. As a device utilizing an electrostatic force, there is a lever type device which includes a cantilever type device and torsion type device. The torsion type device has a torsion bar which extends from the side of a lever for supporting the lever so that the lever displaces utilizing the electrostatic force and a restoring force of torsion of the lever. (see FIG. 6 and FIG. 7)
Such actuators, however, had the following problems. The actuators had very high positioning accuracy, but had a small dynamic range. Because of the small dynamic range, they were able to properly follow surface roughness of the atomic order, whereas it was difficult for the actuators to follow undulation or the like of the substrate, which is macroscopic surface roughness. In addition, there is a possibility to cause contact between the probe and the medium, thus raising problems of occurrence of damages such as a drop in the sensitivity of the probe tip and a decrease in the resolution.
Thus, control is normally carried out using two actuators for coarse movement and fine movement in the conventional apparatus.
In this arrangement, a fine movement mechanism operates for the roughness of the atomic level and a coarse movement mechanism for the relatively great roughness, whereby the probe can trace such great roughness as the undulation of substrate, which could not be covered only by the fine movement mechanism.
As for such control used for observation with STM, Japanese Patent Laid-open Application No. 01-233303 discloses that coarse movement is preferably controlled so that the center of control of fine movement is located at the middle of displacement of the overall fine movement.
The reason is that the roughness configuration can conceivably be captured in more detail in the case of observation of surface configuration when the equal dynamic range is provided in a direction away from the substrate surface.
The conventional control method as disclosed by Japanese Patent Laid-open Application No. 01-233303, however, had the following problem. The conventional control method permitted the probe to trace the pits in the surface configuration, whereas it gave the probe a small movable range in the direction of projections (because the overall movable range was halved between the movable ranges for projections and pits). This resulted in failing to secure a sufficient back amount of the probe for suddenly appearing projections, which caused the probe to happen to touch the projections, thus damaging the probe tip.
The contact or collision often occurs at structures about 10 nm high or more on the surface of the recording medium, for example at portions of grain boundaries etc. between crystals of the medium.
Also, this was a big problem where a crystal metal substrate was used as a substrate of the medium.
In memory systems, a damage on the probe is more serious. If a partial area of the medium were broken, recording and/or reproduction could be carried out by bypassing that area or scattering recording sites as in the case of compact disks presently available. However, the damage on the probe would affect all data to cause errors or incapability of detection unless the probe is cleaned, unless the probe tip is subjected to re-sharpening, or unless the probe is replaced.
Moreover, it is technically difficult to provide a mechanism for recovering the probe, and it is also disadvantageous in respect of the efficiency, taking integration thereof into consideration.
Further, when the electrostatic driving type torsion lever is used as the fine movement mechanism, the restoring force of torsion could not be effectively utilized for retreating the probe in case that the control center is set to the center of the driving range of the fine movement.